This invention concerns improvements in or relating to eyeguards and particularly eyeguards for use with low light level or night vision sights.
A low light level or night vision sight, sometimes referred to as a sighting telescope, has internal illuminating means which provide an observer with a visible image of sufficient brightness for satisfactory viewing. Typically such a sight or telescope comprises an objective lens which focuses such light as is received from a scene or object under low light level conditions onto the photocathode of an image intensifier tube, and an eyepiece or magnifier lens through which an observer views the image of intensified brightness formed on the phosphor of the image intensifier. It is common practice to provide a resilient, usually rubber, eyeguard, sometimes referred to as an eyecup, at the interface of the user's eye area and the eyepiece end of the sight. Such eyeguard provides a relatively comfortable cushion for locating the eye relatively to the sight and setting back the focal distance of the eye with respect to the optical system. The eyeguard may also be adapted to absorb, and hence protect the user's face from the effects of, recoil of a weapon on which the sight is mounted.
Additionally the eyeguard for a low light level or night vision sight usually contains self-closing flaps which block the viewing opening or aperture when the user is not observing through it. Such flaps prevent the escape of light emitted by the internal illuminating means, i.e. usually the image intensifier tube phosphor, when it is switched on but not actually being viewed. Such escape of light is generally undesirable under low light level conditions, and in a military situation at night can be dangerous in that the escaping light may be seen by the enemy.
Generally two overlapping flaps are provided which move from their closed to their open positions by axial deformation of at least a portion of the eyeguard resulting from axial pressure applied by the user's face, in practice by the bony area surrounding the eye viewing through the aperture. On release of such pressure by removal of the face from the eyeguard the collapsible portion expands axially again and returns the flaps to their closed position. An example of an axially actuated two flap system is disclosed in United Kingdom Patent Specification No. 1,264,324.
Axially actuated two flap systems have been found to work well when the applied axial pressure deforms the whole of the collapsible portion of the eyeguard uniformly. This can usually be achieved fairly easily with a hand-held surveillance night sight since the user can readily present the sight to his eye with the sight and eyeguard axis normal to the eye socket, thus ensuring the application of substantially uniform axial pressure round the eyecup. However, this is difficult if not impossible to achieve with, for example, a rifle mounted night sight where the rifle butt constitutes an obstruction and the general firing posture is not well adapted to the facial application of axial pressure uniformly to the eyecup. The angular disparity between the sight and eyeguard axis and the eye socket tends to lead to uneven deformation of the eyeguard and hence only partial opening of one or both flaps. As a result the field of view seen by the eye can be seriously impaired.